Theoretical analysis of control properties for the brushless doubly fed reluctance machine

The brushless doubly fed induction machine (BDFIM) has been extensively researched for approximately 30 years, but a related machine [the brushless doubly fed reluctance machine (BDFRM)], has not. This was mainly due to the fact that reluctance rotor designs were not capable of generating saliency ratios large enough to make the BDFRM competitive with other machines. However, recent developments in reluctance rotors, spurred on by research into synchronous reluctance machines, has resulted in high saliency ratio rotors that are economic to build. This, together with the promise of higher efficiency and simpler control compared to the BDFIM, means that further investigation of the BDFRM is warranted. A relatively limited amount of work to date has been published on the BDFRM. This paper attempts to fill this void by presenting a theoretical analysis of some of the important control properties of the ideal BDFRM.     

Doubly-fed induction motor differential cascade. I. Configurationand analysis in the steady state

In this paper, the configuration and the analysis of a doubly-fed differential cascade (DFDC) in the steady state, is presented. The doubly-fed differential cascade consists of two similar doubly-fed induction machines having their corresponding rotor phases connected. The stators are supplied by voltage phasors of variable frequencies. The rotor recovered slip power of the one machine is used to supply the other machine, through the common rotor connection, maintaining the sum of the two machine speeds constant and equal to the difference of the two stator voltage phasor frequencies. The speed of this differential cascade can be controlled by varying the frequency difference of the two stator voltage phasors. The direction of rotation can be changed by changing the sign of the frequency difference of the stators supply voltages. The set of two doubly-fed induction machines connected in a differential cascade may be used in electrical automobile drive systems     

A doubly-fed induction machine differential drive model for automobiles

In this paper, an electrical drive model, implementing a doubly-fed differential drive (DFDD) is presented. Two doubly-fed induction machines, having the corresponding rotor phases connected, constitutes the differential wheel drive. Two inverters are supplying the machine stators with three-phase power of variable magnitude and frequency. The power required to supply the inverters may be delivered from a constant voltage DC source like a battery. Active power requirements for the machines and the inverters over the DFDD speed range may be obtained. Reactive power requirements for minimum copper loss may be derived as well. The DFDD may provide propulsion to electrical vehicles.     

基于转矩极限的改进风电机组虚拟惯量控制策略

为解决大规模风电并网带来的系统频率稳定性降低问题,风电机组通过虚拟惯量控制可为系统提供短期频率支撑,然而惯性响应期间风电机组转速收敛缓慢,导致一部分转子动能被无故浪费;转速恢复阶段的有功突变易造成频率二次跌落。为此,提出基于转矩极限的改进风电机组虚拟惯量控制策略,实现在释放较少动能的前提下提供与传统策略相同的频率响应服务;并在频率步入准稳态时,借助时变功率函数开始转速恢复,实现转速快速恢复的同时缓解二次频率跌落。基于EMTP-RV仿真软件搭建包含风电场的电力系统模型,验证了所提策略的有效性。     

A complete modeling and simulation of DFIG based wind turbine system using fuzzy logic control

The current paper talks about the variable speed wind turbine generation system (WTGS). So, the WTGS is equipped with a doubly-fed induction generator (DFIG) and two bidirectional converters in the rotor open circuit. A vector control (VC) of the rotor side converter (RSC) offers independent regulation of the stator active and reactive power and the optimal rotational speed tracking in the power maximization operating mode. A VC scheme for the grid-side converter (GSC) allows an independent regulation of the active and reactive power to exchange with the grid and sinusoidal supply currents and keeps the DC-link voltage constant. A fuzzy inference system (FIS) is adopted as an alternative of the conventional proportional and integral (PI) controller to reject some uncertainties or disturbance. The performances have been verified using the Matlab/Simulink software.     

General study of the control principles and dynamic fault behaviour of variable-speed wind turbine and wind farm generic models

The interest towards generic models or sometimes also called standard models of wind turbine generators (WTGs) is significantly increasing. Mainly due to their improved power quality, better controllability and higher power extraction capability, variable-speed wind turbines driving a synchronous or an induction machine are capturing the global market. Throughout this paper, dynamic modelling and performance analysis of the generic models of the variable-speed WTGs, namely the doubly-fed induction generator and the fully-rated converter based WTGs, are achieved using integration between Matlab/Simulink and PSCAD/EMTDC simulation platforms. Later on, the performance of type-4 wind turbine driving a permanent magnet synchronous machine is analysed during fault and then compared with the case when driving a wound rotor induction machine. The differences in control principles and dynamic fault behaviour are highlighted. Afterwards, investigations on wind farm level are accomplished. A case study during which the developed generic models and the generic model of the variable-speed machine are compared is conducted. Different arrangements for the construction of the generic wind farm are considered.     

Redefining the design objectives of large offshore wind turbine rotors

Wind turbine rotors are normally designed such that rotor power coefficient is maximized. Much of this methodology has been inherited from the aviation industry. This paper points out that designing machines for maximum rotor aerodynamic efficiency does not necessarily lead to a lower levelized cost of energy. The argument sits on the premise that levelized cost of energy is strongly influenced by machine capital expenditure (CAPEX) and annual energy production (AEP). We therefore assume that the true design objective is to minimize the CAPEX/AEP ratio. The basis of an alternative design path is presented, which centres on the minimization of total volume of structural material in the wind turbine. This is done whilst maintaining a given rated power. This alternative methodology requires the removal of conventional pre‐set design variables and assumptions which relate to the maximization of rotor power coefficient. We examine how changing chord length, axial induction factor and aerofoil lift coefficient affect material volume in the blade. Following this, we use a custom‐made blade element momentum programme to explore the relative CAPEX of machines with varying design axial induction factor and varying lift coefficient. This relative cost is calibrated to the 5 MW National Renewable Energy Laboratory offshore reference turbine. The effects on the rotor, drivetrain and tower are considered. For a 5 MW offshore machine, it is shown that an overall CAPEX/AEP reduction of over 2% can be achieved by using a low‐induction rotor with blades possessing aerofoils operating at non‐peak lift to drag ratios. This economy is delivered notwithstanding a 2.3% drop in design rotor power coefficient. Copyright © 2014 John Wiley & Sons, Ltd.     

Real-time rotor effective wind speed estimation based on actuator disc theory: Design and full-scale experimental validation

The use of state estimation techniques offers a means of inferring rotor effective wind speed from standard measurements of wind turbines. Typical wind speed estimators rely upon a pre-computed quasi-steady aerodynamic mapping, which describes the relationship between pitch angle and tip-speed ratio and the power coefficient. In practice, the static mapping does not capture the influence of turbine structural dynamics and atmospheric turbulence, inevitably resulting in poor performance of the wind speed estimation. In addition, the turbine aerodynamic properties might not be easily accessible. Thus, this paper presents a rotor effective wind speed estimation method that obviates the requirement for prior knowledge of turbine power coefficients. Specifically, the proposed method exploits a simple actuator disc model, where the aerodynamic power and thrust coefficients can be characterized in terms of axial induction factors. Based on this insight and standard turbine measurements, real-time estimation of rotor effective wind speed and axial induction factors can then be achieved using a simplified turbine drive-train model and an extended Kalman filter. In addition, the actuator disc model can be updated easily over time by calibrating solely two correction factors. Thus, the proposed algorithm presents an alternative for estimating the rotor effective wind speed, which is valuable for numerous applications, for example, LiDAR-assisted control and coherence studies.     

Doubly-fed induction generator drive based WECS using fuzzy logic controller

The purpose of this paper is to improve the control performance of the variable speed, constant frequency doubly-fed induction generator in the wind turbine generation system by using fuzzy logic controllers. The control of the rotor-side converter is realized by stator flux oriented control, whereas the control of the grid-side converter is performed by a control strategy based on grid voltage orientation to maintain the DC-link voltage stability. An intelligent fuzzy inference system is proposed as an alternative of the conventional proportional and integral (PI) controller to overcome any disturbance, such as fast wind speed variation, short grid voltage fault, parameter variations and so on. Five fuzzy logic controllers are used in the rotor side converter (RSC) for maximum power point tracking (MPPT) algorithm, active and reactive power control loops, and another two fuzzy logic controllers for direct and quadratic rotor currents components control loops. The performances have been tested on 1.5 MW doubly-fed induction generator (DFIG) in a Matlab/Simulink software environment.     

Performance analysis of Vernier slotted doubly salient permanent magnet generator for wind power

This paper presents the impact results of the variation of the rotor and the stator tooth pitches (Vernier effect), on the waveform of the back-electromotive force (EMF) generated by the low speed Doubly Salient Permanent Magnet machine (DSPM). The rotating electrical machines with and without Vernier effect are designed and optimized using genetic algorithms combined with finite element method. The optimization of machines parameters is focused on the maximization of the mass to torque ratio. The results show that the machine with Vernier effect has better performances. The obtained Vernier slotted doubly salient permanent magnet generator is then integrated into an autonomous wind energy conversion system. Simulations tests are carried out through Matlab/Simulink. The results show that the proposed machine is a valid and inexpensive alternative for directly coupled wind turbines.     

Estimation of wind misalignment and vertical shear from blade loads

This paper describes the formulation and verification of a novel observer of wind parameters. The general idea behind the proposed approach is to consider the wind turbine rotor as an anemometer. In fact, the rotor responds to varying wind conditions; by properly interpreting this response, one can indirectly measure some desired wind characteristics, as for example yaw and shear, as described here.Measurements of wind conditions obtained this way are not affected by the usual disturbances of existing sensors, for example when installed in the nacelle or in the rotor wake. Furthermore, the approach provides rotor-equivalent quantities, and not the typical point information provided by wind vanes, anemometers or other similar sensors, whose information might be too local for large rotors.The proposed method is here formulated for the observation of wind direction and vertical shear. The new observer is demonstrated first in a comprehensive simulation study using high-fidelity aeroservoelastic models, and then experimentally using an aeroelastically-scaled wind tunnel model.     

Aerodynamic analysis of HAWTs operating in unsteady conditions

This article presents a numerical method for predicting unsteady aerodynamics of horizontal axis wind turbines (HAWTs). In this method the flow field is described by the unsteady incompressible Navier–Stokes equations. The rotor and tower are idealized respectively as actuator disc and flat plate permeable surfaces on which external normal surficial forces are balanced by fluid pressure discontinuities. The external forces exerted by the rotor and tower on the flow are prescribed according to blade element theory. Dynamic behaviour of the rotor aerodynamic characteristics is simulated using either the Gormont or the Beddoes–Leishman model. The resulting mathematical formulation is solved using a control volume finite element method. The fully implicit scheme is used for time discretization. In general, the proposed method has demonstrated its capability to adequately represent the field data. It has been demonstrated that the accuracy of the predicted results depends primarily on the dynamic stall model as well as on the turbulence model employed. Copyright © 2001 John Wiley & Sons, Ltd.     

转子电流混合控制的并网型绕线式异步风力发电机系统

在分析绕线式异步风力发电机系统转子变流器组成特点的基础上,提出一种新型转子电流混合控制的电路拓扑结构及其控制策略。分析了其中双馈运行和斩波调阻运行两种控制方式之间的切换过程和实现连续过渡的控制规律,并对该风力发电系统进行了动态仿真研究。结果表明,该控制方法兼备双馈控制法和斩波调阻法的一些优点,显著降低了转子变流器的硬件成本,实现了两种控制方式之间的连续过渡,可以满足变速恒频风力发电机系统的要求。     

考虑撬棒和直流卸荷保护的双馈电机电磁暂态过程分析

故障期间,撬棒（crowbar）保护和直流卸荷（chopper）保护的工作状态对双馈风电机组（DFIG）的暂态过程有重要影响。为此,针对三相故障不同严重程度下的双馈风力发电系统,综合考虑撬棒保护和直流卸荷保护的协调控制方式,通过分析转子电流、直流母线电压与机端电压跌落的关系,新建统一的保护动作分析判据;在此基础上,分析撬棒保护和直流卸荷保护在不同电压跌落深度下的状态,并建立统一的双馈电机故障后暂态等值模型。最后基于PSCAD/EMTDC提供的1 MW双馈电机标准模型,验证了所建DFIG暂态等值模型的准确性。     

Doubly-fed induction machine differential cascade. II. Verificationby test

In this paper, the doubly-fed differential cascade (DFDC) analysis, in the steady state, given in Part I, is verified by tests. The DFDC consists of two similar doubly-fed machines having the corresponding phases of the two rotors connected. Two sources are used to supply the DFDC stators. One is the mains and the other is a variable frequency converter. The DFDC is loaded with two DC machines acting as brakes. Operations similar to the ones required to provide propulsion to a vehicle are simulated. Variable speed and direction, when the vehicle is moving on a straight course, is simulated by varying the frequency of the frequency converter. Vehicle turn is simulated by distributing the load on the two machines. A nonmoving vehicle, held on a ramp by the DFDC developed torque, is simulated by applying the same frequency to both machines. The experimental results are compared with the results from the analysis     

Rotor dynamics correlation for maximum power and transient control of wind turbines

In this paper, a new rotor dynamics model is developed for transient power output from a horizontal axis wind turbine. In addition to the standard maximum kinetic energy of the wind, the model incorporates rotor velocity and rotational acceleration to enhance the control techniques that convert mechanical to electrical energy via shaft rotation. With current methods, the wind kinetic energy is generally the primary parameter that establishes maximum power output. By relating this wind energy to the rotor dynamics, electrical systems can have a more useful upper bound for the rotor control strategy. The new model predicts the rotor velocity for various turbine configurations, operating over a range of wind conditions. The predicted results show that the same power output is obtained as the standard kinetic energy approach, but with significant additional opportunity to better control the rotor dynamics. Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of Tower Shadow and Wind Turbulence on the Performance of Power System Stabilizers for DFIG-Based Wind Farms

The aim of the paper is to demonstrate the way in which mechanical power variations, due to tower shadow and wind turbulence, influence control performance of power system stabilizer (PSS) loops for doubly-fed induction generators (DFIGs). The PSS auxiliary loops are applied on a specific DFIG control scheme, the flux magnitude and angle controller (FMAC). However, since the PSS signal is applied at the output of the basic controller, the PSS performance characteristics displayed are deemed typical for DFIG control schemes in general. The relative capabilities of PSS controllers based on stator power, rotor speed, and network frequency, when the DFIG turbine is subjected to aerodynamic torque variations, are investigated via simulation studies. A two-generator aggregate model of a wind farm is introduced, which enables the influence of tower shadow and wind turbulence on both an individual turbine and on the overall wind farm itself to be assessed.     

Comparison of direct-drive and geared generator concepts for wind turbines

The objective of this paper is to compare five different generator systems for wind turbines, namely the doubly-fed induction generator with three-stage gearbox (DFIG3G), the direct-drive synchronous generator with electrical excitation (DDSG), the direct-drive permanent-megnet generator (DDPMG), the permanent-magnet generator with single stage gearbox (PMG1G), and the doubly-fed induction generator with single-stage gearbox (DFIG1G). The comparison is based on cost and annual energy yield for a given wind climate. The DFIG3G is a cheap solution using standard components. The DFIG1G seems the most attractive in terms of energy yield divided by cost. The DDPMG has the highest energy yield, but although it is cheaper than the DDSG, it is more expensive than the generator systems with gearbox.     

Technological evolution of onshore wind turbines—a market‐based analysis

Wind energy technology is evolving towards larger machines (longer blades, taller towers and more powerful generators). Scaling up wind turbines is a challenging task, which requires innovative solutions as well as new configurations and designs. The size of wind turbines (in terms of rotor diameter, hub height and rated power) has increased extraordinary from 30 m rotor diameter, 30 m of hub height and 300 kW rated power, usual in the late 1980s, to 92.7 m rotor diameter, 87.7 m of height and 2.1 MW on average at the end of 2014. However, technological evolution has not only been focused on the scaling up process but also on developing innovative solutions that minimize costs at the same time as they deal with aspects of different nature, such as grid code requirements, reliability, quality of the wind resource or prices and availability of certain commodities, among others. This paper analyses the evolution of wind technology from a market‐based perspective by identifying trends in the most relevant technological indicators at the same time as stressing the key differentiating aspects between regions/markets. Evolution and trends in indicators such as rated power, rotor diameter, hub height, specific power, wind class, drive train configuration and power control systems are presented and analysed, showing an intense and fast technological development, which is enabling wind energy to reduce costs and becoming increasingly more competitive with conventional fuel‐based generating technologies. © 2016 The Authors Wind Energy Published by John Wiley & Sons Ltd.     

Whirl flutter analysis of a horizontal-axis wind turbine with a two-bladed teetering rotor

An investigation to explore the possibility of whirl flutter and to find the effect of pitch-flap coupling (δ₃) on teetering motion of the DOE/NASA Mod-2 wind turbine is presented. The equations of motion are derived for an idealized five-degree-of-freedom mathematical model of a horizontal-axis wind turbine with a two-bladed teetering rotor. The model accounts for the out-of-plane bending motion of each blade, the teetering motion of the rotor, and both the pitching and yawing motions of the rotor support. Results show that the Mod-2 design is free from whirl flutter. Selected results are presented indicating the effect of variations in rotor support damping, rotor support stiffnes, and δ₃ on pitching, yawing, teetering, and blade bending motions.